Subacromial Impingement Syndrome

The shoulder joint is the most mobile joint of the body. The anatomical relationship between the humerus (arm bone) and the glenoid of the scapula (the shoulder blade) is a ball and socket joint. However, unlike the hip joint, which gains its stability from a deep socket, the humerus sits on the glenoid, a relatively small and flat “socket.” The acromion and coracoid are two processes that project off of the scapula, providing attachments for ligaments and tendons. The majority of the shoulder’s stability comes from the musculature surrounding it. Subacromial impingement syndrome is the encroachment of subacromial space. The tendons of the supraspinatus (a rotator cuff muscle), the long head of the biceps, and the subacromial bursae pass under the coracoacromial arch. This non-yielding arch confines the subacromial space.

What causes subacromial impingement?

              The basis of subacromial impingement stems from 3 different processes. External compression of the rotator cuff tendons between the humeral head (arm bone) and a portion of the scapula (shoulder blade). Secondarily, the weakening of the supraspinatus, leading to a muscular imbalance and the migration of the humerus into the subacromial space. Lastly, the third process is inflammatory. All three of these processes can be caused by primary or secondary subacromial impingement syndrome.

Primary SIS is caused by structural factors, such as irregular bony anatomy. Secondary SIS is caused by functional factors, such as loss of humeral head depression or stabilization, scapular muscle imbalance, glenohumeral instability, supraspinatus hypertrophy, poor posture, or repetitive overhead movement.  In secondary SIS, the rotator cuff and biceps musculature are compensating for functional instabilities. This compensation of stabilizing the humeral head leads to fatigue and abnormal translation, leading to encroachment of the subacromial space and inflammation of the tendons.  Some associated conditions with SIS include: subacromial bursitis, biceps tendinitis, supraspinatus tendinitis, rotator cuff tears, and tendinosis.1

The concept of impingement was introduced by Neer in 1972. He classified impingement into three categories:

  • Stage I is usually reversible, with swelling and hemorrhage and can occur at any age. However, patients 25 and younger are usually stage I.
  • Stage II consists of structural and cellular changes, such as fibrosis and thickening of the supraspinatus (a rotator cuff muscle), the long head of the bicep’s tendon, and the subacromial bursae. These changes are a result of chronic inflammation. This stage usually occurs in the 25-40-year-old age group.
  • Stage III usually occurs in patients over the age of 40. This stage consists of the more dramatic changes including rotator cuff tears, biceps tendon rupture, and bony changes such as osteophytes. Those who are in stage three typically have a significant tendon degeneration and a history of tendonitis.2


Impingement may arise following a traumatic event. However, pain typically develops over time (weeks to months). Typically, pain is over the anterolateral (the outer-front) of shoulder and radiates to the lateral mid-humerus.  Impingement may also manifest as pain at night, pain with overhead activities and pain while lying on the involved shoulder. Upon history and physical examination by a physician, active and passive range of motion is typically normal, however the patient may experience pain with movements. There may be a marked decrease in strength in rotator cuff musculature. There are two tests with high sensitivity for diagnosing SIS. Neer’s sign is considered positive if there is pain with an internally rotated shoulder that is passively forward flexed to the extent of the range of motion with a fixed scapula. Hawkin’s sign is positive if pain is elicited with passive forward flexion to 90o with maximum internal rotation.


              The main goal of SIS treatment includes decreasing pain and resolving the mechanical problem causing subacromial impingement. The early stages of impingement syndrome can usually be treated relatively conservatively. Conservative treatments may include rest, ice, NSAIDs, physical therapy exercises aimed at training the scapular stabilizers and rotator cuff muscles to function correctly, manual therapy, taping, and possibly a localized drug injection of corticosteroids. Functional limitations such as limiting overhead movements may be necessary to allow the shoulder time to heal.

              If a more aggressive treatment option is necessary because the conservative treatment did not resolve the pain, or a complete rotator cuff tear is visualized on MRI, then it is likely that surgical intervention may be necessary to correct the cause of impingement. The surgical intervention is typically done arthroscopically (surgical scope). Treatment options are something that should be discussed with a physician to ensure the best route is taken.


  1. Koester, Michael, et. al. “Shoulder impingement syndrome.” American Journal of Medicine,

vol. 118, no. 5, May 2005, pp. 452-455., doi:10.1016/j.amjmed.2005.01.040

  1. Dong, Wei, et al. “Treatments for Shoulder Impingement Syndrome.” Medicine, vol. 95,
  2. 10, Mar. 2015, pp. 1–17., doi:10.1097/
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